Recently, resistance to the active ingredients used in insecticide-impregnated ear tags has become a primary problem in the control of horn flies, Haematobia irritans (L.), throughout the Americas and in many other regions of the world. Kunz, S. D. et al., J. Agric. Entomol. 2:358-363 (1985). Many horn fly populations have been shown to be physiologically resistant (10- to 85-fold) to a variety of synthetic pyrethroid insecticides including permethrin, fenvalerate, flucythrinate, cyfluthrin, cypermethrin, and deltamethrin. Studies have also demonstrated the presence of a broad spectrum of cross-resistance to virtually all commercially available pyrethroids as well as to the organochlorine insecticides, DDT and methoxychlor. Byford, R. L. et al., J. Econ. Entomol. 78:768-778 (1985).
This resistance is due to both direct exposure of the horn fly population to specific pyrethroids and to a high level of cross-resistance to insecticides within this class. Byford, R. L. et al., J. Econ. Entomol. 78:768-778 (1985). In particular, the cross-resistance spectrum and results of studies using selected insecticide synergists indicate that pyrethroid-resistance in the horn fly is the result of several mechanisms, including an active site insensitivity similar to knockdown resistance (kdr) and enhanced detoxification. Byford, R. L. et al., J. Econ. Entomol. 78:768-778 (1985) and Sparks, T. C. et al., J. Agric. Entomol. 2:217-233 (1985). In addition, a modified behavior also appears to play a role in the ability of the horn fly to survive in the presence of pyrethroid-impregnated ear tags. Quisenberry, S. S. et al., J. Econ. Entomol. 77:1095-1096 (1984), Lockwood, J. A. et al., Environ. Entomol. 14:873-880 (1985) and Byford, R. L. et al., Environ. Entomol. 16:467-470 (1987a).
A variety of strategies have been proposed to counter resistance in the horn fly including rotation of insecticide classes, use of synergist-insecticide mixtures, regional application of insecticide classes in a "mosaic" pattern, and modification of the delivery systems. Sheppard, D. C. et al., J. Agric. Entomol. 2:317-324 (1985), Sparks, T. C. et al., J. Agric. Entomol. 2:217-233 (1985), Byford, R. L. et al., J. Econ. Entomol. 80:291-296 (1987b) and Byford, R. L. et al., J. Econ. Entomol. 80:111-116 (1987c). It has been shown that the use of other insecticides or synergists (or both) with a pyrethroid often increases the efficacy of the pyrethroid but usually does not eliminate the resistance problem. Typically, some level of resistance to the insecticide combination remains. Byford, R. L. et al., J. Econ. Entomol. 80:111-116. (1987c). Thus, alternative approaches must be found which provide long-lasting horn fly control without elevating resistance to pyrethroid insecticides.
It is therefore desirable to develop a sustained-release insecticide and insecticide device such as an impregnated ear tag, that is effective against pyrethroid-resistant horn flies and other ectoparasites of livestock such as ticks, lice, stable flies, face flies and house flies. It is also desirable to provide a sustained-release insecticide and insecticide device that has a long-term, high degree of efficacy against pyrethroid-resistant horn flies and other ectoparasites of livestock without elevating pyrethoid resistance.